Cardiovascular responses to an orthostatic challenge and electrical-stimulation-induced leg muscle contractions in individuals with paraplegia

Cardiovascular Autonomic Dysfunction in Spinal Cord Injury: Epidemiology, Diagnosis, and Management

Spinal cord injury (SCI) disrupts autonomic circuits and impairs synchronistic functioning of the autonomic nervous system, leading to inadequate cardiovascular regulation. Individuals with SCI, particularly at or above the sixth thoracic vertebral level (T6), often have impaired regulation of sympathetic vasoconstriction of the peripheral vasculature and the splanchnic circulation, and diminished control of heart rate and cardiac output. In addition, impaired descending sympathetic control results in changes in circulating levels of plasma catecholamines, which can have a profound effect on cardiovascular function. Although individuals with lesions below T6 often have normal resting blood pressures, there is evidence of increases in resting heart rate and inadequate cardiovascular response to autonomic provocations such as the head-up tilt and cold face tests. This manuscript reviews the prevalence of cardiovascular disorders given the level, duration and severity of SCI, the clinical presentation, diagnostic workup, short- and long-term consequences, and empirical evidence supporting management strategies to treat cardiovascular dysfunction following a SCI.

Is Technology for Orthostatic Hypotension Ready for Primetime?

Spinal cord injury (SCI) often results in the devastating loss of motor, sensory, and autonomic function. After SCI, the interruption of descending sympathoexcitatory pathways disrupts supraspinal control of blood pressure (BP). A common clinical consequence of cardiovascular dysfunction after SCI is orthostatic hypotension (OH), a debilitating condition characterized by rapid profound decreases in BP when assuming an upright posture. OH can result in a diverse array of insidious and pernicious health consequences. Acute effects of OH include decreased cardiac filling, cerebral hypoperfusion, and associated presyncopal symptoms such as lightheadedness and dizziness. Over the long term, repetitive exposure to OH is associated with a drastically increased prevalence of heart attack and stroke, which are leading causes of death in those with SCI. Current recommendations for managing BP after SCI primarily include pharmacologic interventions with prolonged time to effect. Because most episodes of OH occur in less than 3 minutes, this delay in action often renders most pharmacologic interventions ineffective. New innovative technologies such as epidural and transcutaneous spinal cord stimulation are being explored to solve this problem. It might be possible to electrically stimulate sympathetic circuitry caudal to the injury and elicit rapid modulation of BP to manage OH. This review describes autonomic control of the cardiovascular system before injury, resulting cardiovascular consequences after SCI such as OH, and the clinical assessment tools for evaluating autonomic dysfunction after SCI. In addition, current approaches for clinically managing OH are outlined, and new promising interventions are described for managing this condition.

Characteristics of cardiovascular responses to an orthostatic challenge in trained spinal cord-injured individuals

Background

We investigated cardiovascular responses to an orthostatic challenge in trained spinal cord-injured (SCI) individuals compared to able-bodied (AB) individuals.

Methods

A total of 23 subjects participated, divided into three groups: seven were trained as spinal cord-injured (Tr-SCI) individuals, seven were able-bodied individuals trained as runners (Tr-AB), and nine were untrained able-bodied individuals (UnTr-AB). We measured the cardiovascular autonomic responses in all three groups during each 5-min head-up tilt (HUT) of 0°, 40°, and 80°. Stroke volume (SV), heart rate (HR), and cardiac output (Qc) as cardiovascular responses were measured by impedance cardiography. Changes in deoxyhemoglobin (∆[HHb]) and total hemoglobin (∆[Hb_tot]) concentrations of the right medial gastrocnemius muscle were measured using near-infrared spectroscopy (NIRS).

Results

As the HUT increased from 0° to 80°, Tr-SCI group showed less change in SV at all HUT levels even if HR increased significantly. Mean arterial pressure (MAP) also did not significantly increase as tilting increased from 0° to 80°. Regarding peripheral vascular responses, the alterations of ∆[Hb_tot] from 0° to 80° were less in Tr-SCI group compared to AB individuals.

Conclusion

There is a specific mechanism whereby blood pressure is maintained during a HUT in Tr-SCI group with the elicitation of peripheral vasoconstriction and the atrophy of the vascular vessels in paraplegic lower limbs, which would be associated with less change in SV in response to an orthostatic challenge.

Spinal cord injury-induced cardiomyocyte atrophy and impaired cardiac function are severity dependent

NEW FINDINGS

What is the central question of this study? How does the severity of spinal cord injury affect left ventricular mechanics, function and the underlying cardiomyocyte morphology? What is the main finding and its importance? Here, we show that severe, but not moderate, spinal cord injury causes cardiomyocyte atrophy, altered left ventricular mechanics and impaired cardiac function. The principal aim of the present study was to assess how the severity of spinal cord injury (SCI) affects left ventricular (LV) mechanics, function and underlying cardiomyocyte morphology. Here, we used different severities of T3 spinal cord contusions (MODERATE, 200 kdyn contusion; SEVERE, 400 kdyn contusion; SHAM) and combined standard echocardiography with speckle tracking analyses to investigate in vivo cardiac function and deformation (contractility) after experimental SCI in the Wistar rat. In addition, we investigated changes in the intrinsic structure of cardiac myocytes ex vivo. We demonstrate that SEVERE SCI induces a characteristic decline in LV chamber size and a reduction in in vivo LV deformation (i.e. radial strain) throughout the entire systolic portion of the cardiac cycle [25.6 ± 3.0 versus 44.5 ± 8.1% (Pre-injury); P = 0.0029]. SEVERE SCI also caused structural changes in cardiomyocytes, including decreased length [115.6 ± 7.63 versus 125.8 ± 6.75 μm (SHAM); P = 0.0458], decreased width [7.78 ± 0.71 versus 10.78 ± 1.08 μm (SHAM); P = 0.0015] and an increase in the length/width ratio [14.88 ± 0.66 versus 11.74 ± 0.89 (SHAM); P = 0.0018], which was significantly correlated with LV flow-generating capacity after SCI (i.e. stroke volume, R²  = 0.659; P = 0.0013). Rats with MODERATE SCI exhibited no changes in any metric versus SHAM. This is the first study to demonstrate that the severity of SCI determines the course of changes in the intrinsic structure of cardiomyocytes, which are directly related to contractile function of the LV.

Fundamental hemodynamic mechanisms mediating the response to myocardial ischemia in conscious paraplegic mice: cardiac output versus peripheral resistance

Autonomic dysfunction, a relative sedentary lifestyle, a reduced muscle mass and increased adiposity leads to metabolic abnormalities that accelerate the development of coronary artery disease (CAD) in individuals living with spinal cord injury (SCI). An untoward cardiac incident is related to the degree of CAD, suggesting that the occurrence of a significant cardiac event is significantly higher for individuals with SCI. Thus, understanding the fundamental hemodynamic mechanisms mediating the response to myocardial ischemia has the potential to positively impact individuals and families living with SCI. Accordingly, we systematically investigated if thoracic level 5 spinal cord transection (T₅X; paraplegia) alters the arterial blood pressure response to coronary artery occlusion and if the different arterial blood pressure responses to coronary artery occlusion between intact and paraplegic mice are mediated by changes in cardiac output and or systemic peripheral resistance and whether differences in cardiac output are caused by changes in heart rate and or stroke volume. To achieve this goal, the tolerance to 3 min of coronary artery occlusion was determined in conscious intact and paraplegic mice. Paraplegic mice had an impaired ability to maintain arterial blood pressure during coronary artery occlusion as arterial pressure fell to near lethal levels by 1.38 ± 0.64 min. The lower arterial pressure was mediated by a lower cardiac output as systemic peripheral resistance was elevated in paraplegic mice. The lower cardiac output was mediated by a reduced heart rate and stroke volume. These results indicate that in paraplegic mice, the arterial pressure response to coronary artery occlusion is hemodynamically mediated primarily by cardiac output which is determined by heart rate and stroke volume.

A comparison of passive hindlimb cycling and active upper-limb exercise provides new insights into systolic dysfunction after spinal cord injury

Active upper-limb and passive lower-limb exercise are two interventions used in the spinal cord injury (SCI) population. Although the global cardiac responses have been previously studied, it is unclear how either exercise influences contractile cardiac function. Here, the cardiac contractile and volumetric responses to upper-limb (swim) and passive lower-limb exercise were investigated in rodents with a severe high-thoracic SCI. Animals were divided into control (CON), SCI no exercise (NO-EX), SCI passive hindlimb cycling (PHLC), or SCI swim (SWIM) groups. Severe contusion SCI was administered at the T2 level. PHLC and SWIM interventions began on day 8 postinjury and lasted 25 days. Echocardiography and dobutamine stress echocardiography were performed before and after injury. Cardiac contractile indexes were assessed in vivo at study termination via a left ventricular pressure-volume conductance catheter. Stroke volume was reduced after SCI (91 µl in the NO-EX group vs. 188 µl in the CON group, P < 0.05) and was reversed at study termination in the PHLC (167 µl) but not SWIM (90 µl) group. Rates of contraction were reduced in NO-EX versus CON groups (6,079 vs. 9,225 mmHg, respectively, P < 0.05) and were unchanged by PHLC and SWIM training. Similarly, end-systolic elastance was reduced in the NO-EX versus CON groups (0.67 vs. 1.37 mmHg/µl, respectively, P < 0.05) and was unchanged by PHLC or SWIM training. Dobutamine infusion normalized all pressure indexes in each SCI group (all P < 0.05). In conclusion, PHLC improves flow-derived cardiac indexes, whereas SWIM training displayed no cardiobeneficial effect. Pressure-derived deficits were corrected only with dobutamine, suggesting that reduced β-adrenergic stimulation is principally responsible for the impaired cardiac contractile function after SCI.NEW & NOTEWORTHY This is the first direct comparison between the cardiac changes elicited by active upper-limb or passive lower-limb exercise after spinal cord injury. Here, we demonstrate that lower-limb exercise positively influences flow-derived cardiac indexes, whereas upper-limb exercise does not. Furthermore, neither intervention corrects the cardiac contractile dysfunction associated with spinal cord injury.

A randomized controlled trial on the effects of cycling with and without electrical stimulation on cardiorespiratory and vascular health in children with spinal cord injury

OBJECTIVE

To examine the cardiorespiratory/vascular effects of cycling with and without functional electrical stimulation (FES) in children with spinal cord injury (SCI).

DESIGN

Randomized controlled trial.

SETTING

Pediatric referral hospital.

PARTICIPANTS

Children with SCI (N=30), ages 5 to 13 years, with injury levels from C4 to T11, and American Spinal Injury Association grades A, B, or C.

INTERVENTIONS

Children were randomly assigned to 1 of 3 groups: FES leg cycling exercise, passive leg cycling, or noncycling control group receiving electrical stimulation therapy. After receiving instruction on the use of the equipment, children exercised for 1 hour 3 times per week for 6 months at home with parental supervision.

MAIN OUTCOME MEASURES

Oxygen uptake (Vo(2)) during an incremental arm ergometry test, resting heart rate, forced vital capacity, and a fasting lipid profile.

RESULTS

There were no differences (P>.05) between groups after 6 months of exercise when comparing pre- and postvalues. However, there were differences between groups for some variables when examining percent change. The FES cycling group showed an improvement (P=.035) in Vo(2) (16.2%+/-25.0%) as compared with the passive cycling group (-28.7%+/-29.1%). For lipid levels, the electrical stimulation-only group showed declines (P=.032) in cholesterol levels (-17.1%+/-8.5%) as compared with the FES cycling group (4.4%+/-20.4%).

CONCLUSIONS

Cycling with FES led to gains in Vo(2), whereas electrical stimulation alone led to improvements in cholesterol.

A systematic review of the management of orthostatic hypotension after spinal cord injury

OBJECTIVE

To review systematically the evidence for the management of orthostatic hypotension (OH) in patients with spinal cord injuries (SCIs).

DATA SOURCES

A key word literature search was conducted of original and review articles as well as practice guidelines using Medline, CINAHL, EMBASE, and PsycInfo, and manual searches of retrieved articles from 1950 to July 2008, to identify literature evaluating the effectiveness of currently used treatments for OH.

STUDY SELECTION

Included randomized controlled trials (RCTs), prospective cohort studies, case-control studies, pre-post studies, and case reports that assessed pharmacologic and nonpharmacologic intervention for the management of OH in patients with SCI.

DATA EXTRACTION

Two independent reviewers evaluated the quality of each study, using the Physiotherapy Evidence Database score for RCTs and the Downs and Black scale for all other studies. Study results were tabulated and levels of evidence assigned.

DATA SYNTHESIS

A total of 8 pharmacologic and 21 nonpharmacologic studies were identified that met the criteria. Of these 26 studies (some include both pharmacologic and nonpharmacologic interventions), only 1 pharmacologic RCT was identified (low-quality RCT producing level 2 evidence), in which midodrine was found to be effective in the management of OH after SCI. Functional electrical stimulation was one of the only nonpharmacologic interventions with some evidence (level 2) to support its utility.

CONCLUSIONS

Although a wide array of physical and pharmacologic measures are recommended for the management of OH in the general population, very few have been evaluated for use in SCI. Further research needs to quantify the efficacy of treatment for OH in subjects with SCI, especially of the many other pharmacologic interventions that have been shown to be effective in non-SCI conditions.

Improvements in orthostatic instability with stand locomotor training in individuals with spinal cord injury

Prospective assessment of cardiovascular control in individuals with spinal cord injury (SCI) in response to active stand training. Cardiovascular parameters were measured at rest and in response to orthostatic challenge before and after training in individuals with clinically complete SCI. The goal of this study was to evaluate the effect of active stand training on arterial blood pressure and heart rate and changes in response to orthostatic stress in individuals with SCI. Measurements were obtained in individuals with SCI (n=8) prior to and after 40 and 80 sessions of the standing component of a locomotor training intervention (stand LT). During standing, all participants wore a harness and were suspended by an overhead, pneumatic body weight support (BWS) system over a treadmill. Trainers provided manual facilitation as necessary at the trunk and legs. All individuals were able to bear more weight on their legs after the stand LT training. Resting arterial blood pressure significantly increased in individuals with cervical SCI after 80 training sessions. At the end of the training period, resting systolic blood pressure (BP) in individuals with cervical SCI in a seated position, increased by 24% (from 84 +/- 5 to 104 +/- 7 mmHg). Furthermore, orthostatic hypotension present in response to standing prior to training (decrease in systolic BP of 24 +/- 14 mmHg) was not evident (decrease in systolic BP of 0 +/- 11 mmHg) after 80 sessions of stand LT. Hemodynamic parameters of individuals with thoracic SCI were relatively stable prior to training and not significantly different after 80 sessions of stand LT. Improvements in resting arterial blood pressure and responses to orthostatic stress in individuals with clinically complete cervical SCI occurred following intensive stand LT training. These results may be attributed to repetitive neuromuscular activation of the legs from loading and/or conditioning of cardiovascular responses from repetitively assuming an upright posture.

Non-pharmacological management of orthostatic hypotension after spinal cord injury: a critical review of the literature

STUDY DESIGN

Review.

OBJECTIVES

Identify and describe the body of literature pertaining to non-pharmacological management of orthostatic hypotension (OH) during the early rehabilitation of persons with a spinal cord injury (SCI).

SETTING

Sunnaas Rehabilitation Hospital, Oslo, Norway.

METHODS

SEARCH STRATEGY

a comprehensive search of electronic databases and cited references was undertaken.

SELECTION CRITERIA

case studies, parallel group trials and crossover designs using random or quasi-random assignments were considered. Participants with any level or degree of completeness of SCI and any time elapsed since injury were included. Interventions must have measured at least systolic blood pressure (BP), and have induced orthostatic stress in a controlled manner and have attempted to control OH during an orthostatic challenge.

DATA COLLECTION AND ANALYSIS

studies were selected, assessed and described qualitatively. Meta-analysis was deemed inappropriate.

RESULTS

Four distinct non-pharmacological interventions for OH were identified: application of compression and pressure to the abdominal region and/or legs, upper body exercise, functional electrical stimulation (FES) applied to the legs and biofeedback. Methodological quality varied dramatically between studies. Compression/pressure, upper body exercise and biofeedback therapies have proven inconclusive in their ability to control OH. During orthostatic challenge, FES consistently attenuates the fall in BP; however, its clinical application is less well established.

CONCLUSIONS

The clinical usefulness of compression/pressure, upper body exercise and biofeedback for treating OH has not been proven. FES of the legs holds the most promise.

Circulatory “Efficacy” during progressive aerobic exercise in children: insights from the Q: VO2 relationship

The relationship between circulatory flow (Q) and oxygen uptake (VO2) may provide insights into performance of peripheral mechanisms which govern blood flow during exercise (circulatory efficacy). This study evaluated the response of Q relative to VO2 during progressive upright cycle exercise in a group of 39 preadolescent boys (mean age 12.2 +/- SD 0.5 years). The Q-VO2 relationship was curvilinear, best described by the cubic equation Q = 3.60(VO2)(3) + 5.24(VO2)(2) + 2.40(VO2) - 0.94. Circulatory efficacy, defined as the %DeltaQ/%DeltaVO2 x 100, fell from 70.4% between the first two workloads to 49.7% at peak exercise. This decline in circulatory efficacy is consistent with other published data suggesting a decline in skeletal muscle pump function at high intensity workloads. The pattern of change in relationship of Q and VO2 during progressive exercise in these children is similar to that observed in studies of adults. This implies that performance of peripheral determinants of circulatory responses to exercise is not affected by biological maturation.

Orthostatic hypotension following spinal cord injury: understanding clinical pathophysiology

Motor and sensory deficits are well-known consequences of spinal cord injury (SCI). During the last decade, a significant number of experimental and clinical studies have focused on the investigation of autonomic dysfunction and cardiovascular control following SCI. Numerous clinical reports have suggested that unstable blood pressure control in individuals with SCI could be responsible for their increased cardiovascular mortality. The aim of this review is to outline the incidence and pathophysiological mechanisms underlying the orthostatic hypotension that commonly occurs following SCI. We describe the clinical abnormalities of blood pressure control following SCI, with particular emphasis upon orthostatic hypotension. Possible mechanisms underlying orthostatic hypotension in SCI, such as changes in sympathetic activity, altered baroreflex function, the lack of skeletal muscle pumping activity, cardiovascular deconditioning and altered salt and water balance will be discussed. Possible alterations in cerebral autoregulation following SCI, and the impact of these changes upon cerebral perfusion are also examined. Finally, the management of orthostatic hypotension will be considered.

The effects of lower-extremity functional electric stimulation on the orthostatic responses of people with tetraplegia

OBJECTIVE

To determine whether application of functional electric stimulation (FES) to lower-limb muscles during postural tilting improves orthostatic tolerance in people with tetraplegia.

DESIGN

A crossover design.

SETTING

A rehabilitation hospital.

PARTICIPANTS

Sixteen acute and chronic subjects with tetraplegia (15 men, 1 woman) with complete motor function loss at the C3-7 levels were recruited. Time since injury ranged from 2 to 324 months (mean, 118.9+/-104.2 mo).

INTERVENTION

Subjects were tested on a progressive head-up tilting maneuver with and without the application of FES at 0 degrees , 15 degrees , 30 degrees , 45 degrees , 60 degrees , 75 degrees , and 90 degrees continuously for up to 1 hour. FES was administered to 4 muscle groups including the quadriceps, hamstrings, tibialis anterior, and gastrocnemius muscles bilaterally at an intensity that provided a strong, visible, and palpable contraction. This was to produce a muscle pumping mechanism during the tilting maneuver.

MAIN OUTCOME MEASURES

Systolic blood pressure (SBP), diastolic blood pressure (DBP), heart rate, perceived presyncope score, and the overall duration of orthostatic tolerance, that is, the time that subjects could tolerate the tilting maneuver without developing severe hypotension or other intolerance symptoms.

RESULTS

When the tilt angle was increased, the subjects' SBP and DBP tended to decrease, whereas the heart rate tended to increase in both testing conditions. Adding FES to tilting significantly attenuated the drop in SBP by 3.7+/-1.1 mmHg (P = .005), the drop in DBP by 2.3+/-0.9 mmHg (P = .018), and the increase in heart rate by 1.0+/-0.5 beats/min (P = .039) for every 15 degrees increment in the angle of the tilt. FES increased the overall mean standing time by 14.3+/-3.9 min (P = .003).

CONCLUSIONS

An FES-induced leg muscle contraction is an effective adjunct treatment to delay orthostatic hypotension caused by tilting; it allows people with tetraplegia to stand up more frequently and for longer durations.

Leg vascular resistance increases during head-up tilt in paraplegics

Despite loss of centrally mediated sympathetic vasoconstriction to the legs, spinal cord-injured individuals cope surprisingly well with an orthostatic challenge. This study assessed changes in leg vascular resistance following head-up tilt in healthy (C) and in paraplegic (P) individuals. After 10 min of supine rest, subjects were tilted 30 degrees head-up. Mean arterial pressure (MAP) and total peripheral resistance (TPR) increased in C (MAP from 76.7 +/ -6.6 mmHg to 80.6 +/- 8.2 mmHg; TPR from 1.12 +/- 0.26 AU to 1.19 +/ -0.31 AU) while both remained unchanged in P. Echo Doppler ultrasound determined red blood cell velocity in the femoral artery, which decreased (P from 18.9+/-6.2 cm/s to 12.5 +/- 4.5 cm/s, P = 0.001; C from 16.3 +/- 6.2 cm/s to 10.8 +/- 5.0 cm/s, P = 0.001) and leg vascular resistance, which increased (P from 402 +/- 137 AU to 643 +/- 274 AU, P = 0.001; C from 238 +/- 68 AU to 400 +/- 122 AU, P = 0.003) from supine to upright. The present study shows that independent of supraspinal sympathetic control, humans are able to increase leg vascular resistance and maintain blood pressure during head-up tilt.

The effect of previous weight training and concurrent weight training on endurance for functional electrical stimulation cycle ergometry

Forty-five paraplegic subjects participated in three series of experiments to examine the interrelationships between previous weight training, concurrent weight training and muscle strength and endurance during cycle ergometry elicited by functional electrical stimulation (FES). When subjects only underwent isokinetic weight training (series 1) three times per week on the quadriceps, hamstring and gluteus maximus groups for 12 weeks, strength increased linearly with time for all three muscle groups from an initial average of 17 N to 269 N at the end of training, a 15-fold increase. In the second series of experiments, different groups of subjects either underwent no strength training prior to cycle ergometry or underwent strength training of these same three muscle groups for 2 weeks, four weeks, or 6 weeks prior to cycle ergometry. Any strength training was effective in increasing endurance for cycle ergometry. However, the rate of increase in endurance during cycle ergometry with no prior strength training was only 5 min per week, whereas the rate of increase in cycle endurance during ergometry was 14.6, 25.0, and 33.3 min per week increase in endurance after strength training for 2.4 and 6 weeks, respectively. When weight training was accomplished during FES cycle ergometry (concurrently) in a third series of experiments, there was an additional increase in endurance during cycling if strength training was concurrently accomplished. With no weight training, endurance increased 23 min per week, whereas with concurrent weight training at three times per week, endurance increased during cycling by 41.6 min per week. The results of these experiments seem to show a clear advantage of weight training concurrently and before FES cycle ergometry. Results are given as mean (SD).

Onset of electrical stimulation leg cycling in individuals with paraplegia

PURPOSE

This study investigated cardiovascular and hemodynamic responses during the transition from rest to electrical stimulation-induced leg cycling exercise (ES-LCE) in individuals with chronic paraplegia (PARA).

METHODS

Ten PARA (T(4)-T(9); ASIA A) participated in this study. Heart rate (HR), mean arterial pressure (MAP), stroke volume (SV), and cardiac output (Q) were measured on a beat-to-beat basis at rest and during the first 60 s of ES-LCE.

RESULTS

PARA exhibited two discrete MAP responses during ES-LCE. Those with high thoracic lesions (HIGH: T(4) -T(6), = 5) responded to ES-LCE with a significant rise in MAP (maxdelta 8.3 +/- 3.6 mm Hg), whereas MAP did not exhibit any sustained change from resting values during ES-LCE in those subjects with lower lesions (LOW: T -T, = 5). In HIGH PARA, the immediate increase in MAP corresponded to a decrease in HR (maxdelta 6.8 +/- 3.1 b x min(-1)), which returned toward resting levels by the end of 60 s. In contrast, for LOW PARA there was no change in HR from resting levels during transition to ES-LCE. In both subgroups, SV and Q were not significantly increased during ES-LCE.

CONCLUSION

These results suggest that the on-transient responses of MAP during ES-LCE in HIGH PARA elicited reflex changes in HR via the arterial baroreflex, whereas in LOW PARA, an unchanged HR from rest was likely due to a constant MAP during ES-LCE.

Circulatory hypokinesis and functional electric stimulation during standing in persons with spinal cord injury

OBJECTIVE

To evaluate the effects of functional electric stimulation (FES) of lower limb muscles during 30 minutes of upright standing on the central and peripheral hemodynamic response in persons with spinal cord injury (SCI).

DESIGN

A repeated-measure design. Subjects were used as their own control and underwent 2 testing protocols of FES-augmented standing (active standing) and non-FES standing (passive standing).

SETTING

Rehabilitation hospital.

PARTICIPANTS

Fourteen individuals with SCI (7 with tetraplegia, 7 with paraplegia).

INTERVENTIONS

During active standing, FES was administered to 4 muscle groups of each leg in an overlapping fashion to produce a pumping mechanism during standing. During passive standing, subjects stood for 30 minutes using a standing frame with no FES intervention.

MAIN OUTCOME MEASURES

Central hemodynamic responses of stroke volume, cardiac output, heart rate, arterial blood pressure, total peripheral resistance (TPR), and rate pressure product (RPP) were evaluated by impedance cardiography. All measurements were performed during supine and sitting positions before and after standing, and during 30 minutes of upright standing.

RESULTS

Comparisons between the groups with paraplegia and tetraplegia showed a significant increase in heart rate in the paraplegics after 30 minutes of active standing. During active standing, paraplegics' heart rate increased by 18.2% (p = .015); during passive standing, it increased by 6% (p = .041). TPR in the tetraplegics significantly (p = .003) increased by 54% when compared with the paraplegics during passive standing. Overall, the tetraplegic group had a significantly lower systolic blood pressure (p = .013) and mean arterial pressure (p = .048) than the paraplegics during passive standing. These differences were not detected during active standing. When data were pooled from both groups and the overall groups response to active and passive standing were compared, the results showed that cardiac output, stroke volume, and blood pressure significantly decreased (p < .05) during 30 minutes of passive standing, whereas TPR significantly increased (p < .05). All of the hemodynamic variables were maintained during 30 minutes of active standing, and there were increases in RPP and heart rate after 30 minutes of active standing.

CONCLUSION

FES of the lower extremity could be used by persons with SCI as an adjunct during standing to prevent orthostatic hypotension and circulatory hypokinesis. This effect may be more beneficial to those with tetraplegia who have a compromised autonomic nervous system and may not be able to adjust their hemodynamics to the change in position.